This invention relates to the field of fluid property sensing by which parameters such as fluid level, viscosity and density may be determined for a gas or liquid. The prior art for such devices is extensive, including devices dedicated to liquid level sensing, viscosity determination or density measurement. The present invention relates to the class of sensing devices generally known as resonating sensors. The advantages of resonating sensors have been well documented and several commercial devices exist. These include the vibrating wire viscometer, such as disclosed by James M. Goodwin in Journal Of Physics, Volume 6, 1973; the Dynatrol Viscometer manufactured by Automation Products, Inc.; tuning fork type viscometers, for example, of the type disclosed by M. R. Fisch, et al., J. Acoustic Soc. Am., Volume 60, No. 3, September 1976, p. 623, and tuning fork type liquid level detectors, such as those manufactured by Endress & Hauser and Automation Products, Inc.
These prior art devices have several disadvantages. They require separate transducers in addition to a vibrating structure. They are assemblies of several components requiring exotic materials and/or intricate machining thereby increasing costs. Further, such devices are susceptible to corrosive materials, are larger than may be desired for some applications and have limited sensitivity ranges. Perhaps most significant, is the fact that these devices offer only one function, for example, either viscometry or level detection, but not both.
It is desirable in fluid handling systems to monitor the viscosity of the fluid to ascertain changes in its viscosity which may indicate the need for corrective action. It is also necessary to monitor the level of the fluid, which in the case of liquids, may indicate the need for replenishment. Although there are many applications which have these requirements, one environment for which the present invention in suited, is ink jet printing. In typical commercial ink jet printing systems, an electrically-conductive, oftentimes corrosive, marking fluid or ink is utilized for printing information on products. These may include date codes, bar codes, lot information and the like. Ink jet printers are well known devices and will not be discussed in detail in this disclosure. In order to keep such devices running properly over extended periods of time, the properties of the marking fluid must be monitored and modified when deviations from standard parameters are detected. Thus, the viscosity and/or density of the marking fluid must be monitored as well as the level of the fluid in the various reservoirs in which the marking fluid are kept.
In such systems, it is important to provide measurements of the fluid properties and to do so in an efficient, compact and economical way. Many of the prior art devices mentioned above can perform the desired functions in combination, but cannot do so as efficiently as would be desired.
It is accordingly an object of the present invention to provide for fluid property sensing using apparatus and methods which overcome these shortcomings. According to the present invention, a piezoelectric bender is adapted to this purpose. The term piezoelectric bender is meant to describe a class of devices which are commercially available, such as piezoelectric ceramic wafers which function as transducers between mechanical and electrical energy. One source of such devices is Morgan Matroc, Inc. of Bedford, Ohio. Such devices are often used as audio tone generators and as strain gauges for measuring tensile or compressive force, etc. The present invention adapts these devices to a new purpose not previously known in this art.
Piezoelectric benders, so-called because they are a composite structure which bends when a voltage is applied, and, conversely generate a voltage when strained. One type of bender manufactured by Morgan Matroc is known in the trade as a Bimorph. The Bimorph is a composite structure of two transverse-expander piezoelectric plates bonded together. This structure enhances the bending characteristics of the device and simplifies the electronics required to operate it.
According to the present invention, piezoelectric benders are employed for the dual purpose of detecting liquid level in a fluid system and for monitoring the viscosity and/or density of the fluid. The preferred embodiment accomplishes this by driving the piezoelectric bender at a voltage and frequency which cause it to resonate. By monitoring the output voltage from the bender, it is possible to monitor changes in the viscosity of a liquid or gas. Should the viscosity of such a liquid approach zero, this signifies that the device is no longer immersed in liquid, thereby performing level sensing as well. By measuring the natural resonant frequency instead of output voltage, the density of a liquid can be monitored.
In general, the invention relates to resonator sensors which may be implemented as fluid property sensors by measuring the effects of viscous damping and mass loading on the vibrating structure of the sensor and converting these measurements into the fluid property values of interest via predetermined relationships. These effects can be measured by monitoring an output magnitude and/or frequency. Other suitable techniques include measuring quality factor (Q), logarithmic decrement of free oscillation, decay time constant of free oscillation, electrical impedance characteristics (motional impedance and resistance), drive amplitude required to maintain constant output amplitude, and phase difference between input and output signals.
It is accordingly an object of the present invention to provide a new class of fluid sensing devices which are small, simple in construction, low in cost and which can accurately monitor properties of fluids including liquid level, viscosity and density. It is another object of this invention to provide such a class of devices for use in ink jet printing systems where the properties of liquid inks used in printing must be carefully monitored and altered when variations from desired specifications are detected. These and other objects of the invention will be apparent from the remaining portion of this specification.